1. Field of the Invention
The present invention relates to a method of forming a photo-catalytic film made of titanium oxide which has photo-catalytic activities such as anti-bacteria, anti-pollution, air-cleaning, or hyper hydrophilic property, on a base material and a laminated material including a base on which a photo-catalytic film of titanium oxide is formed. More particularly, it relates to a laminated material including a flexible transparent base on which a photo-catalytic film of titanium oxide is formed.
2. Description of the Related Art
The photo-catalytic activity, the photo-catalyst possessing photo-voltaic force, and the optical semiconductor are a recent focus of attention for their extraordinary application. For example, titanium oxide, which is a photo-catalyst (optical semiconductor), is reported to decompose by oxidation organic stains stuck on a material surface, nitrogen oxides (NOX), sulfur oxides (SOX), air-contaminating substances such as odor sources or bacteria, depending on its oxidizing power based on the photo-catalytic activity. The known embodiments include a method of removing air-contaminating substances with sunlight based on a photo-catalytic catalyst of titanium oxide that is pasted on a building wall (Japanese Published Unexamined Patent Application No. Hei 06-315614), a method of sterilizing bacteria based on a titanium oxide catalyst that is pasted on a wall or a handrail in a hospital (Japanese Published Unexamined Patent Application No. Hei 07-102678), a method of decomposing stains in wastewater based on a titanium oxide catalyst dispersed in the wastewater under the irradiation of light from a UV lamp (Japanese Published Unexamined Patent Application No. Hei 05-92192), a method of applying the self-cleaning action of a photo-catalyst to simplifying the maintenance of a fluorescent lamp or a lighting appliance (Japanese Published Unexamined Patent Application No. Hei 09-129012), and many other applications.
It is reported that the surface of photo-catalytic film becomes highly hydrophilic due to the photo-catalytic reaction. Its application has been widely investigated for prevention of clouding in mirrors (of bathroom or automobile), lenses or windows.
Further, the self-cleaning action of the photo-catalytic coating is well known. In more detail, the photo-catalytic coating formed on building walls, automobile glass, or house windows works to prevent sticking of hydrophobic stains due to its hydrophilic property on the processed surface. Even when some stains are stuck, they are easily decomposed. The hydrophilic property of the coating effects to wash out easily these stains and their decomposition products by rain or water flushing.
The photo-catalytic coating of titanium oxide may be obtained by various production methods. They include a method of applying a hydrolysis product of titanium compounds such as titanium alkoxide or titanium acetate on a base material followed by drying and sintering at more than 500xc2x0 C. to yield a coating of anatase-type titanium oxide, a method of forming amorphous titanium oxide coating on a base by the deposition method followed by annealing at more than 400xc2x0 C. to yield a coating containing anatase-type titanium oxide, a method of oxidizing a surface of metallic titanium at more than 500xc2x0 C. and inducing crystallization, and a method of employing the RF sputtering, which is applied on a base material heated at more than 250xc2x0 C. to yield a coating of anatase-type titanium oxide.
Among these methods, the calcination of a coating of amorphous titanium oxide for the photo-catalytic coating requires heating of the base material at a very high temperature for a long time, which leads economically to a high cost. In addition, formation of a photo-catalytic coating on a plastic resin base is practically impossible in view of the heat resistance of the plastic resin base. The RF sputtering method is an excellent technique for production of anatase-type titanium oxide that yields a large photocurrent. But it requires an expensive production apparatus and is difficult to produce photo-catalytic coating at a low cost. Although some plastic base materials endure the high temperature of 250xc2x0 C., there is no plastic base materials that is equipped with excellent light transmission, heat resistance above 250xc2x0 C., and reasonable production cost simultaneously.
The photo-catalytic coating has lately attracted considerable attention for its photo-voltaic force as well as novel characteristics such as anti-pollution, anti-bacteria, and anti-clouding based on the photochemical reaction on the surface. When a base equipped with a conductive film and a photo-catalytic film is immersed in an aqueous solution or an electrolytic solution under UV light applied on the photo-catalytic film, the photo-voltaic force is generated on the irradiated section. Selective formation of a photo-catalytic film becomes possible using the phenomenon. In more detail, when the above-mentioned base is immersed in an electro-depositing solution containing electro-deposition substances that induce film formation, with or without a bias voltage applied between the conductive film and an opposed electrode, under irradiation on the photo-catalytic film with UV light, the photo-voltaic force is generated on the irradiated section resulting in the electro-deposition of film forming substances on the section. In the case of sufficiently high photo-voltaic force for the photo-catalytic film, the bias voltage may be omitted.
The inventors of the present invention have previously applied for a method of forming extra-fine patterns with excellent resolution using the above-mentioned photo-voltaic force, which may be useful for production of color filters and the like (Japanese Published Unexamined Patent Application Nos. Hei 11-74790, Hei 11-133224, and Hei 11-335894).
With regard to the method of forming semiconductor film on a plastic resin base, Japanese Published Unexamined Patent Application No. Hei 06-11738 describes formation of a crystalline semiconductor film of silicon for an MIM apparatus, in which a film surface of insulating base compound made of silicon is irradiated with energy beam such as laser beam causing meltdown of the surface and converting the surface layer into a film of crystalline silicon. Under the surface layer the insulating base compound layer remains unchanged. Also, Japanese Published Unexamined Patent Application No. Hei 05-315361 describes formation of a semiconductor film on a plastic film, in which a film of non-crystalline substance and an insulating film of metal oxide are formed on a plastic film in this order, followed by application of laser beam on the side of the insulating film causing meltdown of the film of non-crystalline substance near the interface between the non-crystalline film and the insulating film for subsequent crystallization. This method yields a crystalline semiconductor film without giving thermal damages on the plastic film due to laser beam. Further, Japanese Published Unexamined Patent Application No. Hei 05-326402 describes a similar method, in which a heat barrier film and then a film of amorphous silicon are formed on a plastic film to eliminate the thermal effect by laser beam, followed by application of laser beam to induce formation of a polycrystalline silicon film.
All these methods are dependent on crystallization of amorphous semiconductor film by annealing with laser beam. In order to eliminate the thermal effect by laser beam (it sometimes reaches 1,000xc2x0 C.) on the plastic film, one method employs meltdown of only the surface layer of the amorphous semiconductor film, and the other opts for additional formation of a heat barrier layer. With these methods it is impossible to induce crystallization of the whole amorphous film, requiring an additional process providing of a heat barrier layer as well as an expensive laser beam irradiation apparatus. In addition, it becomes difficult to obtain a large-sized film, as the whole surface of the film has to be scanned with the spot of laser beam. They are also disadvantageous because the total crystallization needs much time for completion.
Japanese Published Unexamined Patent Application No. 2000-68520 describes application of UV laser beam of a short wavelength (excimer laser beam) to convert a film of amorphous silicon into semiconductor film of crystalline silicon. This method does not give much thermal effect on the base material, yet the temperature of the base material reaches around 600xc2x0 C. For this reason only glass bases are employed in the invention.
The present invention has been contrived in view of the above problems. The present invention provides a method of forming a photo-catalytic film of titanium oxide on a base material at low temperatures depending on simple processes and a laminated material including a photo-catalytic film of titanium oxide formed on a base material.
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
The present invention also provides a method of forming a photo-catalytic film made of titanium oxide on a base material comprising a process of UV light irradiation on the film in vacuum or in an atmosphere of reducing gas at a temperature maintained between 25 and 300xc2x0 C.
The film of titanium oxide before the irradiation may be a film of amorphous titanium oxide, and the produced photo-catalytic film of titanium oxide may be a film of anatase-type titanium oxide.
The photo-catalytic film of titanium oxide may be a photo-catalytic film of titanium oxide having oxygen deficiencies.
The UV light may be applied with an excimer lamp.
The maintained temperature may range between 50 and 250xc2x0 C.
The base material may be made of plastic, and the maintained temperature ranges between 50xc2x0 C. and a heat resistant temperature of the plastic base material.
The heat resistant temperature of the plastic base material may range between 100 and 230xc2x0 C.
The film of amorphous titanium oxide may be formed by a sputtering method.
The film of amorphous titanium oxide may be formed by an electron beam deposition method.
An anti-reflection film may be set up between the base material and the film of titanium oxide.
The anti-reflection film may be a mono-layer, its refractive index may be a figure between a refractive index of the base material and a refractive index of the photo-catalytic film of titanium oxide, and an optical thickness of the photo-catalytic film, which is defined as a product of the refractive index of the photo-catalytic film times its thickness, may correspond to one-fourth of a central wavelength of visible light, or an integer multiple of one-fourth of a central wavelength of visible light.
The anti-reflection film may be a multi-layer, and its refractive index figures for respective films range between the refractive index of the base material and the refractive index of the photo-catalytic film of titanium oxide, and optical thickness values of the respective anti-reflection film, which are defined as a product of the respective refractive index of each anti-reflection film times its thickness, may correspond to one-fourth of a central wavelength of visible light, or an integer multiple of one-fourth of a central wavelength of visible light.
The refractive index of the anti-reflection film may be between 1.5 and 2.3.
The anti-reflection film may be made of metal oxide.
The anti-reflection film may be made of zirconium oxide.
The anti-reflection film may be made of indium tin oxide (ITO).
The anti-reflection film may be between 50 nm and 100 nm.
This invention also provides a method of forming a photo-catalytic film of titanium oxide, wherein an optical thickness of the photo-catalytic film, which is defined as a product of a refractive index by the film thickness, corresponds to one-half or its integer multiples of a wavelength of light that is near a center of a visible region.
The thickness of the film may be between 70 nm and 150 nm.
The present invention also provides a laminated material including at least a base material, an anti-reflection film, and a photo-catalytic film of titanium oxide arranged in this order.
The present invention also provides a laminated material including at least a base material and a photo-catalytic film of titanium oxide having a thickness between 50 nm and 2.0 xcexcm.
The present invention also provides a laminated material, wherein the base material is a light-transmitting flexible base material made of plastic.
The optical thickness of the photo-catalytic film, which is defined as a product of the refractive index of the photo-catalytic film times its thickness, may correspond to one-half of a central wavelength of visible light, or an integer multiple of one-half of a central wavelength of visible light.
The present invention also provides a flexible photo-catalytic sheet making up a plastic base, an anti-reflection film, and a film of crystalline titanium oxide arranged in this order.
The present invention also provides a flexible photo-catalytic sheet, wherein the sheet is substantially light-transmitting.
The present invention also provides a flexible photo-catalytic sheet making up a plastic base material and a film of anatase-type titanium oxide arranged in this order.